There have been efforts in power electronic design for coupling to a power grid any excess energy that has been tapped from an ecological resource (such as wind or solar energy collectors or some other D.C. source) and also to provide energy storage capability for power grid outages. Outback Power and Xantrex describe DC Coupled system designs. SMA Technologies have an AC coupled design. Apollo Energy and Morningstar Technologies, on information and belief, is developing their own branded AC Coupling. Beacon Power had a 5 KW grid interactive DC coupled device which, on information and belief, has been discontinued by that manufacturer. Princeton University has a 100 KW inverter system that, on information and belief, is the only known commercial class in the field of grid interactive battery backup. Apollo Energy also has an off-grid 3-5 KW class device that allows the ecological source to charge a battery and simulate an AC utility power grid.
TerraWatt power (formerly Advanced Energy Conversion) has developed a product known as the Ecojoule 2500 which may be the first grid-interactive capable, with backup power capable battery-less system. To achieve this functionality, TerraWatt power's ecojoule 2500 internally switches an array of solar cells from a grid feed low capacitance mode to an off-grid high capacitance mode (coupled to an internal super-capacitor (“supercaps”) storage device), thereby enabling power for protected loads during a power outage while the solar cells are energized via the photo-electric effect.
Known approaches suffer from inefficiencies due, at least in part to having multiple power conversions. In a typical grid-interactive with battery backup DC coupled system, the solar array has to be converted to usable power for a battery and then an inverter has to take the usable battery power and ‘invert’ it to usable AC power for the utility. Such feeds can cause the efficiency of an inverter to drop substantially—because even minor fluctuations in battery temperature will affect a battery's ability to hold a charge and therefore affect the amount of available power sent to the AC power grid. This method requires two steps of conversion before the power is exported creating multiple conversion loss.
Accordingly, attempts have been made to use AC coupling in which regular battery-less inverters send power from the ecological source direct to the AC power grid. This does improve the conversion losses; however, when operated off-grid a second inverter presents a false utility grid signal to the first inverter causing it to turn on and then the two inverters work in tandem to charge a battery and maintain a protected AC load service. An AC coupled system is efficient at sending power to the utility, but multiple conversions are presented when it is in off-grid mode and that makes it an inefficient arrangement for sending power to a battery. The invention provides improvements in this art area, particularly, in the management and balance of power between an ecological power source, a grid power connection and the battery.
By having the ecological source on a transfer where it can charge a battery directly or send the charge to the AC power grid depending on the availability of the utility power, the solution provided by the present invention achieves a single conversion step switch in both modes of operation to thereby minimize the conversion losses.